Propelling Ourselves to a Low Carbon Future
Director, impeller and propeller design consultancy
Propellers are an essential and often overlooked component in the engineering field. Known most particularly for their use in aircraft, both fixed and rotary, they are also found in a large number of other applications. These range from the mundane (e.g. small cooling fans in electronic devices) to large, visible installations (e.g. wind turbines); from applications hidden in hydroelectric generators (e.g. Francis Turbines) to screw propellers used for driving much of the world’s shipping across the oceans.
Impellers, the first cousin of propellers, are very similar components and have similar design constraints. The distinguishing feature is that a propeller converts rotary motion into axial thrust. Impellers, in contrast, are used to create a negative, sucking force as part of a pump (see Fig.1).
Fig. 1 Impeller and Propeller
Because propellers essentially turn one type of energy into another, whether as a generator or as a propulsor, their efficiency has a direct bearing on the planet’s carbon footprint. The plethora of applications for propellers and impellers means that any design or practical improvement in their efficiency will have a very positive, environmental impact.
Jon Fraser presented an intriguing and contentious talk at April’s meeting concerning his efforts, over many years, to improve the effectiveness of propellers and impellers. Intriguing because he seemingly offered a relatively easy fix in improving efficiency of propellers and impellers, and contentious because Jon felt unable, at this stage, to reveal the pertinent secrets of his new, augmented design to an eager audience.
Jon’s career experience had been in industrial design at a government level and in retirement as a keen environmentalist in local politics. His background also encompassed memories of his uncles, as RNLI crew members, versed in the nature of the violent seas of Ross and Cromarty, venturing out in appalling conditions to rescue distressed seafarers. These memories had made him particularly sensitive to the needs of the RNLI for a better propulsion system for their boats. He is also convinced of the veracity of good quality turbines, both as alternative energy sources (wind and tidal) and for use in the developing world, most particularly in healthcare, where a dependable and easily accessible source of electricity is essential.
Audio-visual presentation showed the effectiveness of the new propeller design when emplaced in an outboard motor and tested in the sea at Portland. The propeller demonstrated little cavitation and a tight rod of bubbles showed in the wake of the propeller. Cavitation, a significant issue in propeller and impeller operation, is caused by rapid changes in pressure in a liquid leading to the formation of vapour filled cavities where pressure is very low. These collapses and can generating shock waves which can cause wear, tear or fatigue, especially if cyclic. In a naval context, cavitation in a propulsion propeller creates noise, giving away position.
As speed of the test boat increased, the wake narrowed, unlike that from a conventional propeller. Jon described the contrast between a standard and his augmented propeller as that between a blunderbuss and a sniper rifle. Timed, measured runs had shown a fuel saving of 10%, a better turn of speed and greater stability at full throttle. Dynamometer readings with the boat secured to land but engine running in the water showed a 20% increase in thrust. Similarly, in helicopters, his augmented rotors were silent in operation unlike conventional rotors which whistle, loosing energy by cavitation.
Whilst top aerodynamic engineers at Westland’s Augusta helicopters had dismissed his design, the RNLI, Lloyd’s Register and the Wolfson Unit Test Tank were taking the project seriously, as was a major marine engineering company in Southampton (CJR Propulsion). A further twist to Jon’s augmented design provides a tantalising insight into the science of fluid dynamics. Modern fluid dynamics is built on the mathematics of the Swiss mathematician, Daniel Bernoulli (1700-1782) (see Figs. 2 & 3). Although the mathematics has advanced considerably since the 18th century, there remains within this body of work, the so-called ‘Bernoulli Conundrum’ which has not yet been solved and for which there is a prize of £1M for the first correct solution. Jon had an inkling that his augmented propeller design may provide a lead into solving the Bernoulli Conundrum.
At this stage, industry’s response to Jon’s augmented propeller design is mixed. Further test tank trials are continuing in Southampton. In the field of innovation, it is the case that vested interest, sloth, lack of foresight and general disinterest can kill a new invention which should rightly claim a place within current practice (think: Frank Whittle’s development of jet engine, to name but one example). Jon admits that these are significant impediments and he now seeks exposure to peer review where an objective, public and critical appraisal of his designs can be made.
If this is truly the breakthrough in propeller and impeller design Jon believes it is, we will have been privileged at Sherborne Science Cafe that his cutting-edge invention was first raised here!